1. Field of the Invention
The field of the invention is that of compact optical sources capable of emitting in the spectral domains, in which emission is not possible for lack of appropriate laser materials or the difficulty of obtaining them, these sources carrying out their emission by operations of frequency conversion.
Indeed, by using the second-order non-linear susceptibility of certain materials such as lithium niobate (LiNbO.sub.3), it is possible, with an illumination in the near infrared, to emit in the blue region of the spectrum through the phenomenon of frequency doubling.
In the frequency doubling operation, in order that the optical power transfer done on the basis of the incident illumination at the wavelength .lambda. may be efficient, it is necessary, in the material and throughout the interaction, for the non-linear polarization induced by the incident wave to have its phase matched with the wave created at the wavelength 1/2 which it is sought to feed. However, in general, and because of the dispersal of the refraction indices of the material at .lambda..omega. and .lambda..omega./2, this phase-matching condition is not met straight away.
2. Description of the Prior Art
To circumvent this obstacle, a first approach consists in taking advantage of the birefringence of certain materials in order to compensate for the range of variation of their index. There also exists an alternative approach that consists of the use of quasi-phase matching (QPM) applicable in conditions that are far more general than those relating to the birefringence of certain materials. This alternative approach consists in periodically disturbing the nonlinear interaction in order to compensate for the wave vector difference responsible for the phase mismatching.
More precisely, let us take an incident wave being propagated along the axis Ox in the non-linear material and having electrical fields: EQU E.omega.=E.omega.,o e.sup.j(.omega.-.beta..sbsp..omega..sup.x+.phi.P.sbsp.o.sup.)
with a wavelength .lambda..omega.=2.pi..c/.omega. .phi.p.sub.o the phase shift of the pump wave with b.omega. being the constant of propagation of the pump wave
and .beta..sub..omega. =2.pi.n.sub..omega. /.lambda..omega. PA1 with .phi..sub.po as the phase shift of the pump wave, before reflection on the mirror. PA1 where d is the non-linear coefficient brought into play and .epsilon..sub.o is the dielectric permittivity of the vacuum. PA1 with .phi..sub.ho as the phase shift of the pump wave, before reflection on the mirror. PA1 the phase matching condition enabling the cancellation of or compensation for the difference in propagation constant between firstly the non-linear polarization created by the incident wave and, secondly, the harmonic wave generated by this polarization is achieved at the wavelength .lambda.o.sub.a belonging to the set of .lambda.o.sub.i values, in the medium (NLM); PA1 the optical source also includes a dichroic mirror M.sub.1 placed in such a way that the medium (NLM) is contained between the laser and the mirror, said mirror being highly reflective at the wavelength .lambda.oi and highly transparent at the wavelength .lambda.o.sub.i /2 in such a way as to reinject a light wave at .lambda.o.sub.a into the laser, the energy level of which at the wavelength .lambda.oa is highly depleted with respect to the energy levels at the wavelengths .lambda.oi.noteq..lambda.oa.
c: the velocity of light in vacuum PA2 .omega.: frequency.
In an appropriate material, this wave may give rise to a second-order non-linear polarization written as follows: EQU P.sub.NL =.epsilon..sub.o dE.sub..omega..sup.2 =.epsilon..sub.o dE.sub..omega.,o.sup.2 e.sup.j(2 .omega.t-2.beta..sbsp..omega..sup.x2.phi..sbsp.po.sup.)
This polarization radiates a wave at double frequency liable, as and when the interaction takes place, to set up a harmonic beam with a half wavelength .lambda.2.omega.=.lambda..omega./2 and a propagation constant b2.omega. with b.sub.2.omega. =2.pi.n.sub.2.omega. /.lambda.2.omega. where n.sub.2.omega. is the refraction index of the material at the wavelength .lambda.2.omega.. The electrical field corresponding to this wave may be written as follows: EQU E.sub.2.omega. =E.sub.2.omega.,o e.sup.j(2.sbsp..omega.t.sup.-.beta..sbsp.2.omega..sup.x+.phi..sbsp.ho.sup. )
It can thus be seen that the phase shift .LAMBDA..phi. between the non-linear polarization that forms the source of the radiation at .lambda.2.omega. and the harmonic wave that is to be fed constructively by means of this polarization will play a decisive role in the conversion .lambda..omega..fwdarw..lambda.2.omega.. In fact, this phase shift is expressed at the end of a distance x of interaction as follows: EQU .DELTA..phi.=(.beta.2.omega.-2.beta..omega.)x=.DELTA..beta.x
with EQU .DELTA..beta.=4.pi.(n.sub.2.omega. -n.omega.).lambda..omega.=4.pi..DELTA.n/.lambda..omega.
It can clearly be seen that, because of the range of variation of the indices, this phase shift is generally not zero.
However, it is possible to create a periodic variation .DELTA..beta.=m.K or K=2.pi./.LAMBDA. with .LAMBDA. as the period of the disturbance and .LAMBDA.=2L.sub.C if L.sub.C is the length of coherence corresponding to the interaction distance at the end of which the polarization and harmonic wave have accumulated a .pi. phase shift.
The disturbance may be introduced into any parameter coming into play in the non-linear interaction (refraction index, dispersion of the indices, non-linear coefficient brought into play, etc.).
Through this periodic disturbance, the phase shift Deltaf between polarization and harmonic wave is reduced by .pi. at the end of each L.sub.C, namely instead of continuing to accumulate, it is reduced to zero at each coherence length. In this respect, FIG. 1 illustrates the three possible examples: curve a) phase mismatching, curve b): quasi-phase matching, curve c): perfect phase matching.
The object of the invention relates to a source using a laser emitting an instant wave at .lambda..omega. so as to generate a wave .lambda.2.omega. by means of a frequency doubler, the frequency doubler being a non-linear medium (NLM) in which the phase matching condition or the quasi-phase matching condition is achieved at the wavelength .lambda..omega..
The source according to the invention enables the problem of the spectral width of emission of the laser to be resolved. This phenomenon is pronounced in the laser diode context whereas the phase matching conditions or quasi-phase matching conditions are met in the medium (NLM) only for certain very precise wavelengths.
For this purpose, the compact source according to the invention uses the "locking" of the emission length of the laser by the injection, into this wave, of a signal with given wavelengths.
The invention is based on the following observation which has been arrived at empirically:
When a laser diode is used to pump a waveguide laser of the Nd:MgO:LiNbO.sub.3 type described in the reference (E. Lallier, J. P. Pocholle, M. Papuchon, M. de Micheli, M. J. Li, Q. He, D. B. Ostrowsky, C. Grezes-Besset and E. Pelletier, "Nd:MgO:LiNbO.sub.3 channel waveguide laser devices", IEEE J. Quantum Electron. 27 (3), pp. 618-625, 1991), the laser diode locks its transmission wavelength automatically to the value corresponding to the maximum absorption of neodymium in LiNbO.sub.3.
This means that it is possible to lock the emission wavelength of a laser emitting an incident wave in a range of wavelengths .LAMBDA.o.sub.i to the wavelength .LAMBDA.o.sub.a by the reinjection, into said laser, of a light wave whose energy level at the wavelength .LAMBDA.o.sub.a is highly depleted as compared with the energy levels of the wavelength .LAMBDA.o.sub.i .noteq..LAMBDA.o.sub.a.
Thus, the invention makes use of this phenomenon to lock the emission of the laser used to supply the frequency doubling medium (NLM) at the wavelength .LAMBDA.o.sub.a for which the phase matching condition or quasi-phase matching condition is met within said medium (NLM).
Indeed, in the frequency doubling operation, the harmonic power generated is directly taken from the power of the pump wave and, when the conversion efficiency becomes considerable, the depletion of the pump wave itself becomes appreciable, in this case at the wavelength .LAMBDA.o.sub.a.